Process for the production of aromatic amines

ABSTRACT

A process for the production of aromatic amines which comprises the reduction of an aromatic compound containing at least two nitro groups with hydrogen in the presence of a catalyst is disclosed. The reaction is carried out so as to maintain up to 10% by weight of reaction intermediates (aminonitrites) and the solution produced is treated to recover the aromatic amine with a purity of over 99% and a mixture consisting of aromatic amine and reaction intermediates is recycled to the reactor.

The present application is a National Phase filing of the InternationalApplication No. PCT/EP01/07478 with a filing date of 29 Jun. 2001, whichclaims priority to Italian Application MI2000A001457 filed 29 Jun. 2000.

This invention relates to a process for the production of an aromaticamine. More specifically, the invention relates to a continuous processfor the production of an aromatic diamine, especiallymetatoluenediamine.

Processes for producing aromatic amines by the hydrogenation of thecorresponding nitro-compounds are known for example from DE 1,542,544,BE 631,964, FR 1,359,438, GB 768,111, and U.S. Pat. Nos. 3,935,264,4,387,247, 4,717,774, 5,563,296, 5,728,880 and 5,849,947.

According to these processes, aromatic derivatives containing two ormore nitro groups are reacted with hydrogen, or another reducing gassuch as hydrogen sulfide, at a high temperature and under pressure inthe presence of catalysts selected from heavy transition metals such asiron, cobalt, nickel, copper, silver or caesium, used as such, on inertcarriers, or in the form of oxides, hydroxides or carbonates. Thereaction may be carried out in the presence of solvents, generallywater, aliphatic alcohols or their mixtures, and, optionally, in thepresence of reduction adjuvants of the nitro group to the amine groupsuch as carbon monoxide. The reaction is generally carried out instirred reactors where the catalyst, in a finely subdivided form, iskept in suspension and is extracted in continuous fashion together withthe amines produced.

A drawback of at least some of these processes for producing aromaticdi- or poly-amines, such as metatoluenediamine, is that they produce anend-product in which there are numerous reaction by-products which arealso formed as a result of the operating conditions of the reactors, inparticular high temperature and pressure. These reaction by-products, aswell as reducing the yield to useful product, also pollute the desiredproduct. The lack of purity of the product is undesirable with regard todownstream processing and may have a serious effect on the running ofproduction plants.

U.S. Pat. No. 5,563,296 discloses a process for the production ofaromatic amines which, although carried out at a high temperature,generally between 120 and 220° C., is said to significantly reduce theformation of secondary reactions. This process comprises feeding anaromatic compound containing two or more nitro groups, hydrogen and thecatalyst to a “Jet Loop” type reactor where the mixing effect isguaranteed by a Venturi tube whose diffuser is immersed in the reagentsuspension. The latter, in turn, is continuously extracted by means of apump and is partially recycled to the Venturi tube as a device forsucking the reagents and mixing them with the catalyst dispersed in thesuspension. The suspension extracted from the bottom of the reactor iscooled in a heat exchanger outside the reaction container, before beingrecycled to the Venturi tube, to eliminate the reaction heat.

The Applicant has now found a process for the preparation of an aromaticamine which, without the necessity of having to resort to particulartypes of reactor, as in the case of the process disclosed in U.S. Pat.No. 5,563,296, allows a product to be recovered from a traditionalreaction container having a high level of purity.

The object of the present invention therefore relates to a process forthe production of an aromatic amine which comprises:

-   a) feeding an aromatic compound having at least two nitro groups to    a reaction zone and contacting the compound with a catalyst;-   b) passing, preferably in a continuous stream, a reducing gas    through the reaction zone to contact the gas with the said compound;-   c) reducing the nitro groups into amine groups in the presence of    the catalyst, the catalyst comprising a supported active metal,    until a conversion degree is reached at which the level of reaction    intermediates is 30% or less by weight of the aromatic amine,    preferably 10% or less, for example from 5 to 10%;-   d) discharging a first stream from the reaction zone and recycling    the first stream to the reaction zone;-   e) discharging a second stream comprising the desired aromatic amine    product and 30% or less, preferably 10% or less, for example from 5    to 10% by weight based on the aromatic amine of reaction    intermediates from the reaction zone;-   f) recovering from the stream of step (e) the aromatic amine at a    purity of over 99% and a mixture containing at least 50% of aromatic    amine and up to 50% by weight of reaction intermediates; and-   g) feeding the mixture of step (f) to the reaction zone.

Preferably, the process of the invention is continuous.

Suitably, the reaction zone is defined by a reaction container equippedwith means to agitate the components in the reaction zone, for example astirrer.

Desirably, the first stream is discharged from the bottom of thereaction zone. The first reaction stream is suitably recycled to thereaction zone by means of a continuous, closed recycle loop. Prior tore-entering the reaction zone, heat is preferably removed from the firststream by heat exchange so as to regulate the temperature in thereaction zone.

The catalyst may be in any suitable form for contact with the aromaticcompound to catalyze reduction of the compound but is preferably finelydivided, more preferably finely sub-divided.

In a preferred embodiment, the catalyst is in the form of a suspensioncomprising a finely subdivided catalyst dispersed in a reaction medium.Preferably the reaction medium comprises water. Suitably, the reducinggas is bubbled through the suspension and contacted with the aromaticcompound.

Preferably, in step e) of the process of the invention, the secondstream is removed from the reaction zone by means of a filter cartridgeimmersed in the suspension comprising the catalyst.

The reducing gas suitably comprises hydrogen and in a preferredembodiment is substantially pure hydrogen.

The intermediate produced during the reduction reaction is typically anaminonitrite, that is a compound that has only been partly reduced fromthe aromatic reagent.

The process for the production of an aromatic amine of the presentinvention is particularly suitable for the production ofmetatoluenediamine starting from dinitrotoluene. In this case, a stirredreactor (CSTR) or a column reactor fed at the head with thedinitrotoluene reagent and at the bottom with the reducing gas by meansof a micro-bubble diffuser may be employed as desired. In such a columnreactor in a reaction medium the reagents contact in a countercurrentmanner with the reagent descending slowly owing to the micro-bubbles ofthe reducing gas which are rising, and the reducing gas itself.

In the case of dinitrotoluene, during the reaction, the dinitrotolu n isfirst reduced to amino-nitrotoluene. In the case of 2,4-dinitrotoluene,the intermediate comprises 4-amino-2-nitrotoluene and2-amino-4-nitrotoluene isomers then to metatoluenediamine. To obtain apartially reduced product, it is preferable to operate with molar ratiosof reducing gas to aromatic compound, for example,hydrogen/dinitrotoluene from 5 to 10.

Suitably, the suspension discharged from the bottom of the reactioncontainer is practically without high-boiling by-products. It ispreferably continuously extracted, for example by means of a pumpcapable of processing a suspension, subjected to cooling in a heatexchanger to eliminate or reduce the reaction heat, and subsequentlyrecycled to the reactor. The recycling operation advantageously enablesthe temperature to be regulated inside the reactor and also favours thehomogenization of the suspension and therefore effects a better contactbetween the reagents and catalyst.

In addition to the reduction of dinitrotoluene to metatoluenediamine,the present process is also suitable for the reduction of other aromaticcompounds containing nitro groups such as dinitrobenzene, and itscorresponding isomers, and dinitroxylenes.

Any catalyst capable of catalyzing the reduction of nitro groups toamine groups can be used in the process of the present invention.Examples of catalysts which are particularly suitable for the purposeare metals belonging to group VIII of the Periodic Table such as iron,nickel, cobalt, ruthenium, palladium and platinum. Preferably thecatalyst is supported on an inert material such as alumina or silica.The especially preferred catalyst according to the present invention ispalladium supported on carbon.

The catalyst suitably is in the form of particles with an averagedimension from 10 to 100 μm. The catalyst desirably contains the GroupVIII metal, for example palladium, at a level from 0.5 to 20% w/w, withrespect to the total weight of the catalyst.

Suitably, the recycling flow-rate of the first stream in step (d), andits temperature are regulated so as to maintain a temperature from 120to 220° C. in the reaction zone. The pressure inside the reaction zoneis suitably from 0.1 to 10 MPa and preferably from 0.1 to 5 MPa.

The second stream is preferably fed to a system for the recovery of thearomatic amine, for example to a distillation process, preferably adistillation column under vacuum. To avoid or reduce the formation ofheavy products, the distillation process suitably is operated undervacuum, at a pressure from 10 to 20 mmHg and at a temperature, at thebottom in the case of a column, of 150 to 160° C.

As an alternative to distillation, the process of the present inventionmay comprise feeding the second stream discharged from the reaction zoneto a crystallization unit where the aromatic amine is crystallized, forexample at a temperature of 90 to 110° C. and is subsequently recoveredin a manner known to those skilled in the art. In both cases,distillation under vacuum and crystallization, the remaining mixturecontaining at least 50% of aromatic amine and up to 50% by weight ofreaction intermediates, preferably 50/50 is recycled to the reactionzone.

The invention is illustrated by the following non-limiting examples.

EXAMPLE 1

18,000 kg/h of 2,4-dinitrotoluene was fed to the upper part of a CSTRstirred reaction container having a volume of 30 m³, and 1,187 kg/h ofhydrogen was fed close to the bottom, by means of a bubble distributor.

An aqueous suspension containing 1% by weight of solid catalystconsisting of carbon particles on which 5% by weight of palladium isdistributed was inside the reactor. The particles of catalyst had anaverage dimension of 30 μm and a surface area of 900 m²/g.

A temperature of 135° C. and a pressure of 0.4 MPa was maintained insidethe reactor.

90 m³/h of suspension was discharged from the bottom of the reactor,cooled in an external reactor to 90° C. and recycled to the head of thereactor. An aqueous solution containing 90% of 2,4-toluenediamine (TDA)and 10% of 2-amino-4-nitrotoluene (AMN) was extracted by means of filtercartridges immersed in the suspension and was fed to a distillationcolumn operating at 10 mmHg and at a temperature of 150° C. at thebottom.

11.945 Kg/h of 2,4-toluenediamine (TDA), having a purity of over 99%,was discharged from the head of the column, and an aqueous solutioncontaining AMN and TDA (50/50) from the bottom, which was recycled tothe reactor.

EXAMPLE 2

The aqueous solution extracted by means of the filter cartridges of thereactor as per Example 1, having a composition of 90% of TDA, 8% of AMNand 2% of water, was fed to a static drop film crystallizer operating ata temperature of about 90 to 110° C.

The feed (10 parts) to the crystallizer was separated into 8 parts ofTDA and 2 parts of a mixture of TDA, AMN and water recycled to thereactor. To reach a purity of over 99%, the crystallized product (TDA)was washed with 1 part of water, also recycled to the reactor.

1. A process for the production of an aromatic amine which comprises: a)feeding an aromatic compound having at least two nitro groups to areaction zone and contacting the compound with a catalyst, wherein thecatalyst is in the form of a suspension within the reaction zone; b)passing a reducing gas through the reaction zone to contact the gas withthe said compound; c) reducing the nitro groups into amine groups in thepresence of the catalyst, the catalyst comprising a supported activemetal, until a conversion degree is reached at which the level ofreaction intermediates is 30% or less by weight of the aromatic amine;d) discharging a first stream from the reaction zone and recycling thefirst stream to the reaction zone; e) discharging a second streamcomprising the desired aromatic amine product and 30% or less by weightbased on the aromatic amine of reaction intermediates from the reactionzone; f) recovering from the stream of step (e) the aromatic amine at apurity of over 99% and a mixture containing at least 50% of aromaticamine and up to 50% by weight of reaction intermediates; and g) feedingthe mixture of step (f) to the reaction zone.
 2. A process according toclaim 1 which is continuous and in which the level of the reactionintermediate in step c) and e) is 10% or less.
 3. A process according toclaim 1 in which heat is removed from the first stream by heat exchangeso as to regulate the temperature in the reaction zone.
 4. The processaccording to claim 1 in which the catalyst is finely divided.
 5. Aprocess according to claim 4 in which the catalyst is in the form of asuspension comprising a finely divided catalyst dispersed in a reactionmedium.
 6. A process according to claim 5 in which the reaction mediumcomprises water.
 7. A process according to claim 5 in which the reducinggas is bubbled through the suspension and contacted with the aromaticcompound and, in step e), the second stream is removed from the reactionzone by means of a filter cartridge immersed in the suspensioncomprising the catalyst.
 8. A process according to claim 7 in which thereducing gas comprises hydrogen.
 9. A process according to claim 1 inwhich the aromatic compound containing at least two nitro groups isdinitrotoluene.
 10. A process according to claim 1 in which the molarratios of reducing gas to aromatic compound is from 5:1 to 10:1.
 11. Aprocess according to claim 1 in which the catalyst is selected frommetals belonging to group VIII of the Periodic Table.
 12. A processaccording to claim 11 in which the catalyst is palladium supported oncarbon.
 13. A process according to claim 12 in which the catalyst is inthe form of particles with an average dimension from 10 to 100 μm.
 14. Aprocess according to claim 1 in which the temperature in the reactionzone is from 120 to 220° C. and the pressure is from 0.5 to 10 MPa. 15.A process according to claim 1 in which the second stream is fed to adistillation process for the recovery of the aromatic amine.
 16. Aprocess according to claim 15 in which the second stream discharged fromthe reaction zone is fed to a crystallization unit from which thearomatic amine is recovered.